In the field of the lithographic printing plate precursor, metal supports are widely used. Above all, it is known that an oxide film can be formed when a direct electric current is turned on in an acid solution with an aluminum sheet as the anode, which is generally known as the Alumite treatment, in addition, aluminum is light weight and inexpensive. Aluminum oxide film formed on the surface of aluminum by Alumite treatment is high in acid resistance and hardness as compared with metallic aluminum, further, many minute holes called pores are formed regularly on the surface of the film structure, and the surface area by BET method (Brunauer-Emmett-Teller gas adsorption equation) steeply increases due to the pores, and so very advantageous in the improvements of hydrophilicity and the adhering property when a film is formed.
In the formation of pores, an anodic oxide film is conventionally formed in an electrolyte by using a proper electrolyte, such as a sulfuric acid, and applying a direct electric current at constant current to the support. The sectional form of the pore of the anodic oxide film obtained in such a method can be seen by observing the broken-out section of the pore with an ultra-high resolution scanning electron microscope (SEM) (S-900). In that case, the form of the pore is generally a pipe-like form in which the diameters at the surface mouth part and at the maximum diameter part are almost the same.
Dyes, which are light-sensitive layer components, sometimes enter into such pores of the anodic oxide film and cause a phenomenon of residual color that the dyes remain even after development, and binders, which are also light-sensitive layer components, cause a phenomenon of residual film that the binders remain even after development. Therefore, techniques of sealing the pores by appropriate process to prevent the light-sensitive layer from entering into the pores according to necessity have been known.
Further, in recent years, lithographic printing plate precursors for heat mode CTP (computer to plate) capable of image-forming by exposure in near infrared to infrared region, in particular, capable of direct plate-making from digital data of, e.g., a computer, by recording an image by utilizing the heat generated by irradiation with a laser having emission region in near infrared to infrared region attract public attention.
These are lithographic printing plate precursors which convert irradiated laser beam for imaging to heat by using a light-to-heat converting material (also, simply called “a light/heat converting material”), change the solubility in a developing solution of a light-sensitive layer by the generated heat, or heat-decomposed a light-sensitive layer, or subject a light-sensitive layer to explosive abrupt removal (ablation) by sudden heating. When aluminum is used as the support of these lithographic printing plate precursors for heat mode CTP (hereinafter simply referred to as “heat mode photographic material”), since the heat conductivity of aluminum is high, abrupt heat release to the support side occurs and exothermic loss is generated, which is one of the cause of the reduction of sensitivity. Conversely speaking, it is expected that if the heat insulation of the support surface can be improved and heat release can be suppressed to the minimum, sensitivity can be increased by that portion.
On the other hand, techniques for higher sensitization by using organic materials having low heat conductivity, such as PET, as a support have been tried variously, but organic materials are low in hydrophilicity as compared with metallic materials and dimensional accuracy is deteriorated during printing by absorbing moisture, and so these materials cannot be used for printing of a high degree, e.g., color printing and highly precise printing in the present state.
As the support for a heat mode photographic material, it has been an object of the industry to improve the only drawback of aluminum of heat insulating property with making the best use of excellent aspects, e.g., easiness of various surface treating processes, hydrophilicity, and the stability of dimensional accuracy.
For improving the heat insulating property of aluminum, there are a method of thickening the thickness of an anodic oxide film by making use of the property of low heat conductivity of the anodic oxide film itself, and a method of increasing the void ratio of an anodic oxide film by immersing an anodic oxide film formed in an acid aqueous solution or an alkali aqueous solution to enlarge the pore diameter. However, when a film thickness is thickened, extra quantity of electricity of that portion is required, which causes the increase of production costs and, further, the pore is deepened by the increment of the thickness, and so the residual color is liable to be noticeable. Contrary to this, when a pore diameter is enlarged, the void ratio in the anodic oxide film on the support surface is improved and the heat insulating property is improved, and the heat generated by irradiation with laser beams is difficult to dissipate on the aluminum sheet side of the metal. As a result, the generated heat effectively functions in a light-sensitive layer, which results in high sensitivity. On the other hand, when a pore diameter is enlarged, a light-sensitive layer and a coloring dye for absorbing laser beams contained in a light-sensitive layer enter into the pores and cause a residual color and a residual film. For reducing a residual color, by performing sealing treatment such as boiling water sealing by mixing a very small amount of a sealing agent, the surface of an anodic oxide film is covered, thereby the specific surface area is reduced, and at the same time, a light-sensitive layer does not enter into the pores, thus residual color is improved. However, when pore density is reduced as a result of sealing treatment to cover the surface of an anodic oxide film, pile driving effect that a light-sensitive layer enters into the pores is reduced and an adhesive force is decreased. In addition, when pores are enlarged, adjacent pores form a multi-pore, which leads to extreme lowering of the anodic oxide film strength, and results in smearing due to peeling off of the film and also the deterioration of press life.
With the advancement of the image-forming technique in recent years, a technique of direct plate-making by scanning a printing plate with a diaphragmed laser beam and directly forming a character original and an image original on the printing plate without using a film original has attracting public attention.
As such an image-forming material, a so-called thermal type positive lithographic printing plate precursor in which an infrared absorber contained in a heat-sensitive layer exhibits a light/heat converting function and generates heat when subjected to exposure, and the exposed area of the heat-sensitive layer is solubilized in an alkali due to the heat generated by exposure and forms a positive image, and a thermal type negative lithographic printing plate precursor in which a radical and an acid are generated by a radical generating agent and an acid generating agent due to that heat, thereby a radical polymerization reaction and an acid crosslinking reaction progress and the exposed area of the heat-sensitive layer is insolubilized to form a negative image are exemplified. That is, in such thermal type image formation, a heat is generated by a light/heat converting material in a heat-sensitive layer by irradiation with a laser beam and that heat causes an image-forming reaction.
However, since the heat conductivity of a support is extremely high as compared with that of a heat-sensitive layer in an aluminum support on which a surface-roughened anodic oxide film is formed, the heat generated on the periphery of the interface of the heat-sensitive layer and the support diffuses to the inside of the support before being used sufficiently in image formation, as a result, the following phenomena occur in the interface of the heat-sensitive layer and the support.
In the first place, in a positive type heat-sensitive layer, there is the problem of low sensitivity, i.e., when a heat diffuses to the inside of a support and an alkali solubilization reaction of the heat-sensitive layer becomes insufficient, a residual film is generated on the area to be a non-image area originally, and this is an essential problem of the positive type heat-sensitive layer.
Further, in such a thermal type positive lithographic printing plate precursor, infrared absorbers having a function of light/heat conversion is requisite, but these compounds are low in solubility due to comparatively large molecular weights, further they are adsorbed onto the micro pores formed by the anodic oxidation and it is difficult to removed them, and so there is another problem that a residual film is liable to be generated in a developing process by an alkali developing solution.
On the other hand, in a negative type heat-sensitive layer, when a heat diffuses to the inside of a support and the insolubilization in a developing solution of the heat-sensitive layer on the periphery of the interface of the heat-sensitive layer and the support becomes insufficient, an image is not formed sufficiently on the area to be an image area originally and the image is removed during development, or easily peeled off during printing, even if an image is formed.
Further, in recent years, various investigations and suggestions have been done on a lithographic printing plate precursor capable of being mounted on a printing machine after exposure as it is and performing printing. For example, a lithographic printing plate precursor of forming an image by the coalescence of fine particles by heat has been proposed.
However, in such a lithographic printing plate precursor, sensitivity is low due to the dissipation of heat to an aluminum support, and the strength of the image area of a heat-sensitive layer becomes weak when the coalescence of fine particles is insufficient, which causes an insufficient press life.
For coping with these problems, it has been tried to enlarge the micro pores of an anodic oxide film for the purpose of inhibiting the heat generated in a heat-sensitive layer from diffusing to an aluminum support.
Further, from the similar viewpoint, sealing of micro pores has been made on trial by the method of immersing an aluminum support comprising an aluminum sheet having provided thereon an anodic oxide film in a boiling water or a boiling aqueous solution containing an inorganic or organic salt, or the method of exposing an aluminum support to a water vapor bath.
However, in the method of enlarging the micro pores of the anodic oxide film, the sensitivity and the press life are improved but the smearing resistance (i.e., also, called “the stain resistance”) is deteriorated. “Smearing resistance” here means the property of hardly generating smearing on the non-image area when printing is stopped in the middle of printing, and printing is restarted from the stare of the lithographic printing plate being left as it is on the printing machine. On the other hand, in the method of sealing the micro pores, the smearing resistance is improved but the sensitivity and the press life are deteriorated. Accordingly, sufficiently satisfactory level is not achieved yet in either case.